Arrival rate throttles for workload management

ABSTRACT

A computer-implemented method, system and article of manufacture for managing workloads in a computer system, comprising monitoring system conditions and operating environment events that impact on the operation of the computer system, regulating execution of one or more queries based on the monitored system conditions and operating environment events using arrival rate throttles, and performing the queries to access data in a database for presentation and use to a user.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending and commonly-assigned applications:

United States Utility patent application Ser. No. 10/730,348, filed Dec. 8, 2003, by Douglas P. Brown, Anita Richards, Bhashyam Ramesh, Caroline M. Ballinger and Richard D. Glick, and entitled Administering the Workload of a Database System Using Feedback;

United States Utility patent application Ser. No. 10/786,448, filed Feb. 25, 2004, by Douglas P. Brown, Bhashyam Ramesh and Anita Richards, and entitled Guiding the Development of Workload Group Definition Classifications;

United States Utility patent application Ser. No. 10/889,796, filed Jul. 13, 2004, by Douglas P. Brown, Anita Richards, and Bhashyam Ramesh, and entitled Administering Workload Groups;

United States Utility patent application Ser. No. 10/915,609, filed Aug. 10, 2004, by Douglas P. Brown, Anita Richards, and Bhashyam Ramesh, and entitled Regulating the Workload of a Database System;

United States Utility patent application Ser. No. 11/468,107, filed Aug. 29, 2006, by Douglas P. Brown and Anita Richards, and entitled A System and Method for Managing a Plurality of Database Systems, which applications claims the benefit of U.S. Provisional Patent Application Ser. No. 60/715,815, filed Sep. 9, 2005, by Douglas P. Brown and Anita Richards, and entitled A System and Method for Managing a Plurality of Database Systems;

U.S. Provisional Patent Application Ser. No. 60/877,977, filed on Dec. 29, 2006, by Douglas P. Brown and Anita Richards, and entitled Managing Events in a Computing Environment;

United States Utility patent application Ser. No. 11/716,889, filed on Mar. 12, 2007, by Douglas P. Brown, Anita Richards, Mark Morris and Todd A. Walter, and entitled Virtual Regulator for Multi-Database Systems, which application claims the benefit of U.S. Provisional Patent Application Nos. 60/877,766, 60/877,767, 60/877,768, and 60/877,823, all of which were filed Dec. 29, 2006;

United States Utility patent application Ser. No. 11/716,892, filed on Mar. 12, 2007, by Douglas P. Brown, Scott Gnau and Mark Morris, and entitled Parallel Virtual Optimization, which application claims the benefit of U.S. Provisional Patent Application Nos. 60/877,766, 60/877,767, 60/877,768, and 60/877,823, all of which were filed Dec. 29, 2006;

United States Utility patent application Ser. No. 11/716,880, filed on Mar. 12, 2007, by Mark Morris, Anita Richards and Douglas P. Brown, and entitled Workload Priority Influenced Data Temperature, which application claims the benefit of U.S. Provisional Patent Application Nos. 60/877,766, 60/877,767, 60/877,768, and 60/877,823, all of which were filed Dec. 29, 2006;

United States Utility patent application Ser. No. 11/716,890, filed on Mar. 12, 2007, by Mark Morris, Anita Richards and Douglas P. Brown, and entitled Automated Block Size Management for Database Objects, which application claims the benefit of U.S. Provisional Patent Application Nos. 60/877,766, 60/877,767, 60/877,768, and 60/877,823, all of which were filed Dec. 29, 2006; and

United States Utility patent application Ser. No. 11/803,248, filed on May 14, 2007, by Anita Richards and Douglas P. Brown, and entitled States Matrix for Workload Management Simplification;

all of which applications are incorporated by reference herein.

BACKGROUND

Modern computing systems execute a variety of requests concurrently and operate in a dynamic environment of cooperative systems, each comprising of numerous hardware components subject to failure or degradation.

The need to regulate concurrent hardware and software “events” has led to the development of a field which may be generically termed “Workload Management.” For the purposes of this application, “events” comprise, but are not limited to, one or more signals, semaphores, periods of time, hardware, software, business requirements, etc.

Workload management techniques focus on managing or regulating a multitude of individual yet concurrent requests in a computing system by effectively controlling resource usage within the computing system. Resources may include any component of the computing system, such as CPU (central processing unit) usage, hard disk or other storage means usage, or I/O (input/output) usage.

Workload management techniques fall short of implementing a full system regulation, as they do not manage unforeseen impacts, such as unplanned situations (e.g., a request volume surge, the exhaustion of shared resources, or external conditions like component outages) or even planned situations (e.g., systems maintenance or data load).

Many different types of system conditions or events can impact negatively the performance of requests currently executing on a computer system. These events can remain undetected for a prolonged period of time, causing a compounding negative effect on requests executing during that interval. When problematic events are detected, sometimes in an ad hoc and manual fashion, the computing system administrator may still not be able to take an appropriate course of action, and may either delay corrective action, act incorrectly or not act at all.

A typical impact of not managing for system conditions is to deliver inconsistent response times to users. For example, often systems execute in an environment of very cyclical usage over the course of any day, week, or other business cycle. If a user ran a report near standalone on a Wednesday afternoon, she may expect that same performance with many concurrent users on a Monday morning. However, based on the laws of linear systems performance, a request simply cannot deliver the same response time when running stand-alone as when it runs competing with high volumes of concurrency.

Therefore, while rule-based workload management can be effective in a controlled environment without external impacts, it fails to respond effectively when those external impacts are present.

SUMMARY

In general, in one aspect, the invention features a method for managing workloads in a computer system, comprising regulating execution of one or more queries using arrival rate throttles, and performing the queries to access data in a database for a user.

The arrival rate throttles provide a capability to throttle back and forward the execution of the queries so as to manage workload concurrency levels. The regulating step or means queues the queries before execution and releases the queries for execution using the arrival rate throttles. The arrival rate throttles identify how many queries are executed for a particular workload in a specified amount of time. In addition, the method or system may include a step or means for regulating execution of the queries using concurrency throttles in conjunction with the arrival rate throttles.

In another aspect, the invention features a database system for accessing a database. The database system includes means for regulating execution of one or more queries using arrival rate throttles, and means for performing the queries to access data in a database for a user.

In yet another aspect, the invention features a computer program, stored on a tangible storage medium, for use in performing the method steps of regulating execution of one or more queries using arrival rate throttles, and performing the queries to access data in a database for a user.

Other features and advantages will become apparent from the description and claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a node of a database system.

FIG. 2 is a block diagram of a parsing engine.

FIG. 3 is a flow chart of a parser.

FIGS. 4-7 are block diagrams of a system for administering the workload of a database system.

FIG. 8 is a flow chart of an event categorization and management system.

FIG. 9 illustrates how conditions and events may be comprised of individual conditions or events and condition or event combinations.

FIG. 10 is a table depicting an example rule set and working value set.

FIG. 11 illustrates an n-dimensional matrix that is used to perform with automated workload management.

DETAILED DESCRIPTION

The event management technique disclosed herein has particular application to large databases that might contain many millions or billions of records managed by a database system (“DBS”) 100, such as a Teradata Active Data Warehousing System available from NCR Corporation. FIG. 1 shows a sample architecture for one node 105 ₁ of the DBS 100. The DBS node 105 ₁ includes one or more processing modules 110 ₁ . . . _(N), connected by a network 115 that manage the storage and retrieval of data in data storage facilities 120 ₁ . . . _(N). Each of the processing modules 110 ₁ . . . _(N) may be one or more physical processors or each may be a virtual processor, with one or more virtual processors running on one or more physical processors.

For the case in which one or more virtual processors are running on a single physical processor, the single physical processor swaps between the set of N virtual processors. Each virtual processor is generally termed an Access Module Processor (AMP) in the Teradata Active Data Warehousing System.

For the case in which N virtual processors are running on an M processor node, the node's operating system schedules the N virtual processors to run on its set of M physical processors. If there are 4 virtual processors and 4 physical processors, then typically each virtual processor would run on its own physical processor. If there are 8 virtual processors and 4 physical processors, the operating system would schedule the 8 virtual processors against the 4 physical processors, in which case swapping of the virtual processors would occur.

Each of the processing modules 110 ₁ . . . _(N) manages a portion of a database that is stored in a corresponding one of the data storage facilities 120 ₁ . . . _(N). Each of the data storage facilities 120 ₁ . . . _(N) includes one or more disk drives. The DBS 100 may include multiple nodes 105 ₂ . . . _(N) in addition to the illustrated node 105 ₁, connected by extending the network 115.

The system stores data in one or more tables in the data storage facilities 120 ₁ . . . _(N). The rows 125 ₁ . . . _(Z) of the tables are stored across multiple data storage facilities 120 ₁ . . . _(N) to ensure that the system workload is distributed evenly across the processing modules 110 ₁ . . . _(N). A parsing engine 130 organizes the storage of data and the distribution of table rows 125 ₁ . . . _(Z) among the processing modules 110 ₁ . . . _(N). The parsing engine 130 also coordinates the retrieval of data from the data storage facilities 120 ₁ . . . _(N) in response to queries received from a user at a mainframe 135 or a client computer 140. The DBS 100 usually receives queries in a standard format, such as SQL.

In one example system, the parsing engine 130 is made up of three components: a session control 200, a parser 205, and a dispatcher 210, as shown in FIG. 2. The session control 200 provides the logon and logoff function. It accepts a request for authorization to access the database, verifies it, and then either allows or disallows the access.

Once the session control 200 allows a session to begin, a user may submit a SQL request that is routed to the parser 205. As illustrated in FIG. 3, the parser 205 interprets the SQL request (block 300), checks it for proper SQL syntax (block 305), evaluates it semantically (block 310), and consults a data dictionary to ensure that all of the objects specified in the SQL request actually exist and that the user has the authority to perform the request (block 315). Finally, the parser 205 runs an optimizer (block 320) that develops the least expensive plan to perform the request.

The DBS 100 described herein accepts performance goals for each workload as inputs, and dynamically adjusts its own performance, such as by allocating DBS 100 resources and throttling back incoming work. In one example system, the performance parameters are called priority scheduler parameters. When the priority scheduler is adjusted, weights assigned to resource partitions and allocation groups are changed. Adjusting how these weights are assigned modifies the way access to the CPU, disk and memory is allocated among requests. Given performance objectives for each workload and the fact that the workloads may interfere with each other's performance through competition for shared resources, the DBS 100 may find a performance setting that achieves one workload's goal but makes it difficult to achieve another workload's goal.

The performance goals for each workload will vary widely as well, and may or may not be related to their resource demands. For example, two workloads that execute the same application and DBS 100 code could have differing performance goals simply because they were submitted from different departments in an organization. Conversely, even though two workloads have similar performance objectives, they may have very different resource demands.

The system includes a “closed-loop” system management (CLSM) architecture capable of satisfying a set of workload-specific goals. In other words, the system is a goal-oriented workload management system capable of supporting complex workloads and capable of self-adjusting to various types of workloads. In Teradata, the workload management system is generally referred to as Teradata Active System Management (TASM).

The system's operation has four major phases: 1) assigning a set of incoming request characteristics to workload groups, assigning the workload groups to priority classes, and assigning goals (called Service Level Goals or SLGs) to the workload groups; 2) monitoring the execution of the workload groups against their goals; 3) regulating (adjusting and managing) the workload flow and priorities to achieve the SLGs; and 4) correlating the results of the workload and taking action to improve performance. The performance improvement can be accomplished in several ways: 1) through performance tuning recommendations such as the creation or change in index definitions or other supplements to table data, or to recollect statistics, or other performance tuning actions, 2) through capacity planning recommendations, for example increasing system power, 3) through utilization of results to enable optimizer self-learning, and 4) through recommending adjustments to SLGs of one workload to better complement the SLGs of another workload that it might be impacting. All recommendations can either be enacted automatically, or after “consultation” with the database administrator (DBA).

The system includes the following components (illustrated in FIG. 4):

1) Administrator (block 405): This component provides a GUI to define workloads and their SLGs and other workload management requirements. The administrator 405 accesses data in logs 407 associated with the system, including a query log, and receives capacity planning and performance tuning inputs as discussed above. The administrator 405 is a primary interface for the DBA. The administrator also establishes workload rules 409, which are accessed and used by other elements of the system.

2) Monitor (block 410): This component provides a top level dashboard view, and the ability to drill down to various details of workload group performance, such as aggregate execution time, execution time by request, aggregate resource consumption, resource consumption by request, etc. Such data is stored in the query log and other logs 407 available to the monitor. The monitor also includes processes that initiate the performance improvement mechanisms listed above and processes that provide long term trend reporting, which may including providing performance improvement recommendations. Some of the monitor functionality may be performed by the regulator, which is described in the next paragraph.

3) Regulator (block 415): This component dynamically adjusts system settings and/or projects performance issues and either alerts the DBA or user to take action, for example, by communication through the monitor, which is capable of providing alerts, or through the exception log, providing a way for applications and their users to become aware of, and take action on, regulator actions. Alternatively, the regulator 415 can automatically take action by deferring requests or executing requests with the appropriate priority to yield the best solution given requirements defined by the administrator (block 405).

The workload management administrator (block 405), or “administrator,” is responsible for determining (i.e., recommending) the appropriate application settings based on SLGs. Such activities as setting weights, managing active work tasks and changes to any and all options will be automatic and taken out of the hands of the DBA. The user will be masked from all complexity involved in setting up the priority scheduler, and be freed to address the business issues around it.

As shown in FIG. 5, the workload management administrator (block 405) allows the DBA to establish workload rules, including SLGs, which are stored in a storage facility 409, accessible to the other components of the system. The DBA has access to a query log 505, which stores the steps performed by the DBS 100 in executing a request along with database statistics associated with the various steps, and an exception log/queue 510, which contains records of the system's deviations from the SLGs established by the administrator. With these resources, the DBA can examine past performance and establish SLGs that are reasonable in light of the available system resources. In addition, the system provides a guide for creation of workload rules 515 which guides the DBA in establishing the workload rules 409. The guide accesses the query log 505 and the exception log/queue 510 in providing its guidance to the DBA.

The administrator assists the DBA in: a) Establishing rules for dividing requests into candidate workload groups, and creating workload group definitions. Requests with similar characteristics (users, application, table, resource requirement, etc) are assigned to the same workload group. The system supports the possibility of having more than one workload group with similar system response requirements. b) Refining the workload group definitions and defining SLGs for each workload group. The system provides guidance to the DBA for response time and/or arrival rate threshold setting by summarizing response time and arrival rate history per workload group definition versus resource utilization levels, which it extracts from the query log (from data stored by the regulator, as described below), allowing the DBA to know the current response time and arrival rate patterns. The DBA can then cross-compare those patterns to satisfaction levels or business requirements, if known, to derive an appropriate response time and arrival rate threshold setting, i.e., an appropriate SLG. After the administrator specifies the SLGs, the system automatically generates the appropriate resource allocation settings, as described below. These SLG requirements are distributed to the rest of the system as workload rules. c) Optionally, establishing priority classes and assigning workload groups to the classes. Workload groups with similar performance requirements are assigned to the same class. d) Providing proactive feedback (i.e.: Validation) to the DBA regarding the workload groups and their SLG assignments prior to execution to better assure that the current assignments can be met, i.e., that the SLG assignments as defined and potentially modified by the DBA represent realistic goals. The DBA has the option to refine workload group definitions and SLG assignments as a result of that feedback.

The internal monitoring and regulating component (regulator 415), illustrated in more detail in FIG. 6, accomplishes its objective by dynamically monitoring the workload characteristics (defined by the administrator) using workload rules or other heuristics based on past and current performance of the system that guide two feedback mechanisms. It does this before the request begins execution and at periodic intervals during query execution. Prior to query execution, an incoming request is examined to determine in which workload group it belongs, based on criteria as described in more detail below. Concurrency or arrival rate levels, i.e., the numbers of concurrent executing queries from each workload group, or the rate at which they have been arriving, are monitored, and if current workload group levels are above an administrator-defined threshold, a request in that workload group waits in a queue prior to execution until the level subsides below the defined threshold. Query execution requests currently being executed are monitored to determine if they still meet the criteria of belonging in a particular workload group by comparing request execution characteristics to a set of exception conditions. If the result suggests that a request violates the rules associated with a workload group, an action is taken to move the request to another workload group or to abort it, and/or alert on or log the situation with potential follow-up actions as a result of detecting the situation. Current response times and throughput of each workload group are also monitored dynamically to determine if they are meeting SLGs. A resource weight allocation for each performance group can be automatically adjusted to better enable meeting SLGs using another set of heuristics described with respect to FIG. 6.

As shown in FIG. 6, the regulator 415 receives one or more requests, each of which is assigned by an assignment process (block 605) to a workload group and, optionally, a priority class, in accordance with the workload rules 409. The assigned requests are passed to a workload query (delay) manager 610, which is described in more detail with respect to FIG. 7. The regulator 415 includes an exception monitor 615 for detecting workload exceptions, which are recorded in a log 510.

In general, the workload query (delay) manager 610 monitors the workload performance from the exception monitor 615, as compared to the workload rules 409, and either allows the request to be executed immediately or places it in a queue for later execution, as described below, when predetermined conditions are met.

If the request is to be executed immediately, the workload query (delay) manager 610 places the requests in buckets 620 _(a) . . . _(s) corresponding to the priority classes to which the requests were assigned by the administrator 405. A request processor function performed under control of a priority scheduler facility (PSF) 625 selects queries from the priority class buckets 620 _(a) . . . _(s), in an order determined by the priority associated with each of the buckets 620 _(a) . . . _(s), and executes it, as represented by the processing block 630 on FIG. 6.

The PSF 625 also monitors the request processing and reports throughput information, for example, for each request and for each workgroup, to the exception monitor 615. Also included is a system condition monitor 635, which is provided to detect system conditions, such as node failures. The exception monitor 615 and system monitor 635 collectively define an exception attribute monitor 640.

The exception monitor 615 compares the throughput with the workload rules 409 and stores any exceptions (e.g., throughput deviations from the workload rules) in the exception log/queue 510. In addition, the exception monitor 615 provides system resource allocation adjustments to the PSF 625, which adjusts system resource allocation accordingly, e.g., by adjusting the priority scheduler weights. Further, the exception monitor 615 provides data regarding the workgroup performance against workload rules to the workload query (delay) manager 610, which uses the data to determine whether to delay incoming requests, depending on the workload group to which the request is assigned.

As can be seen in FIG. 6, the system provides two feedback loops. The first feedback loop includes the PSF 625 and the exception monitor 615. In this first feedback loop, the system monitors, on a short-term basis, the execution of requests to detect deviations greater than a short-term threshold from the defined service level for the workload group to which the requests were defined. If such deviations are detected, the DBS 100 is adjusted, e.g., by adjusting the assignment of system resources to workload groups.

The second feedback loop includes the workload query (delay) manager 610, the PSF 625 and the exception monitor 615. In this second feedback loop, the system monitors, on a long-term basis, to detect deviations from the expected level of service greater than a long-term threshold. If it does, the system adjusts the execution of requests, e.g., by delaying, swapping out or aborting requests, to better provide the expected level of service. Note that swapping out requests is one form of memory control in the sense that before a request is swapped out it consumes memory and after it is swapped out it does not. While this is the preferable form of memory control, other forms, in which the amount of memory dedicated to an executing request can be adjusted as part of the feedback loop, are also possible.

In addition to enforcing PSF 625 rules to achieve resource management goals, the present invention provides the capability to throttle back incoming work so as to keep the particular workload concurrency levels low, and not misallocate shared resources, such as AWTs, spool, etc., from higher priority workloads. However throttling based on concurrency does not always give the DBA all the power, granularity or dynamics needed to properly manage workload resources. This is especially a problem when a workload is comprised of a heterogeneous mix of requests, e.g., some being short, others being long, or in an environment of frequent arrival rate surges and lulls.

To solve this problem, the present invention introduces arrival rate workload throttles (also known simply as arrival rate throttles) at 645, wherein the PSF 625 regulates execution of the queries using the arrival rate throttle techniques employed by the workload query (delay) manager 610. The arrival rate throttles 645 provide a capability to throttle back and forward the execution of the queries so as to manage workload concurrency levels.

Rather than queuing and releasing incoming requests based on concurrency levels alone, the regulator 415 queues the queries before execution and releases the queries for execution, using the arrival rate throttles based on a workload's arrival rate. The arrival rate throttles identify how many queries are executed for a particular workload in a specified amount of time, e.g., a user-configurable period, and can be used by the regulator 415 in conjunction with concurrency throttles. This provides the ability to more finely tune the amount of incoming work than can be provided with concurrency throttles.

For example, if the arrival rate throttle definition allows 10 queries per minute, and each query takes on average 2 seconds to execute, 20 out of 60 seconds in that minute would perform that workload. Concurrency throttles, on the other hand, may only allow one concurrent query, meaning that there is always work present for the full 60 seconds. The arrival rate throttle allows the flow of incoming work to decrease by a factor of 3 without completely shutting off the flow of incoming work.

The workload query (delay) manager 610, shown in greater detail in FIG. 7, receives an assigned request as an input. A comparator 705 determines if the request should be queued or released for execution. It does this by determining the workload group assignment for the request and comparing that workload group's performance against the workload rules, provided by the exception monitor 615. For example, the comparator 705 may examine the concurrency level of requests being executed under the workload group to which the request is assigned. Further, the comparator may compare the workload group's performance against other workload rules.

If the comparator 705 determines that the request should not be executed, it places the request in a queue 710 along with any other requests for which execution has been delayed. The comparator 705 continues to monitor the workgroup's performance against the workload rules and when it reaches an acceptable level, it extracts the request from the queue 710 and releases the request for execution. In some cases, it is not necessary for the request to be stored in the queue to wait for workgroup performance to reach a particular level, in which case it is released immediately for execution.

Once a request is released for execution it is dispatched (block 715) to priority class buckets 620 _(a) . . . _(s), where it will await retrieval by an AMP Worker Task (AWT) 625.

The exception monitor 615, receives throughput information from the AWT 625. A workload performance to workload rules comparator 705 compares the received throughput information to the workload rules and logs any deviations that it finds in the exception log/queue 510. It also generates the workload performance against workload rules information that is provided to the workload query (delay) manager 610.

As stated above, a series of AMP Worker Task (AWT) process the requests. An AWT is a thread/task that runs inside of each virtual AMP. An AWT is generally utilized to process requests/queries from users, but may also be triggered or used by internal database software routines, such as deadlock detection.

Pre-allocated AWTs are assigned to each AMP and work on a queue system. That is, each AWT waits for work to arrive, performs the work, and then returns to the queue and waits for more work. Due to their stateless condition, AWTs respond quickly to a variety of database execution needs. At the same time, AWTs serve to limit the number of active processes performing database work within each AMP at any point in time. In other words, AWTs play the role of both expeditor and governor of requests/queries.

AMP worker tasks are one of several resources that support the parallel performance architecture within the Teradata database. AMP worker tasks are of a finite number, with a limited number available to perform new work on the system. This finite number is an orchestrated part of the internal work flow management in Teradata. Reserving a special set of reserve pools for single and few-AMP queries may be beneficial for active data warehouse applications, but only after establishing a need exists. Understanding and appreciating the role of AMP worker tasks, both in their availability and their scarcity, leads to the need for a more pro-active management of AWTs and their usage.

AMP worker tasks are execution threads that do the work of executing a query step, once the step is dispatched to the AMP. They also pick up the work of spawned processes, and of internal tasks such as error logging or aborts. Not being tied to a particular session or transaction, AMP worker tasks are anonymous and immediately reusable and are able to take advantage of any of the CPUs. Both AMPs and AWTs have equal access to any CPU on the node. A fixed number of AWTs are pre-allocated at startup for each AMP in the configuration, with the default number being 80. All of the allocated AWTs can be active at the same time, sharing the CPUs and memory on the node.

When a query step is sent to an AMP, that step acquires a worker task from the pool of available AWTs. All of the information and context needed to perform the database work is contained within the query step. Once the step is complete, the AWT is returned to the pool. If all AMP worker tasks are busy at the time the message containing the new step arrives, then the message will wait in a queue until an AWT is free. Position in the queue is based first on work type, and secondarily on priority, which is carried within the message header. Priority is based on the relative weight that is established for the PSF 625 allocation group that controls the query step. Too much work can flood the best of databases. Consequently, all database systems have built-in mechanisms to monitor and manage the flow of work in a system. In a parallel database, flow control becomes even more pressing, as balance is only sustained when all parallel units are getting their fair portion of resources.

The Teradata database is able to operate near the resource limits without exhausting any of them by applying control over the flow of work at the lowest possible level in the system. Each AMP monitors its own utilization of critical resources, AMP worker tasks being one. If no AWTs are available, it places the incoming messages on a queue. If messages waiting in the queue for an AWT reach a threshold value, further message delivery is throttled for that AMP, allowing work already underway to complete. Other AMPs continue to work as usual.

One technique that has proven highly effective in helping Teradata to weather extremely heavy workloads is having a reasonable limit on the number of active tasks on each AMP. The theory behind setting a limit on AWTs is twofold: 1) that it is better for overall throughput to put the brakes on before exhaustion of all resources is reached; and 2) keeping all AMPs to a reasonable usage level increases parallel efficiency. However this is not a reasonable approach in a dynamic environment.

Ideally, the minimum number of AWTs that can fully utilize the available CPU and I/O are employed. After full use of resources has been attained, adding AWTs will only increase the effort of sharing. As standard queuing theory teaches, when a system has not reached saturation, newly-arriving work can get in, use its portion of the resources, and get out efficiently. However, when resources are saturated, all newly-arriving work experiences delays equal to the time it takes someone else to finish their work. In the Teradata database, the impact of any delay due to saturation of resources may be aggravated in cases where a query has multiple steps, because there will be multiple places where a delay could be experienced.

In one particular implementation of the Teradata database, 80 (eighty) is selected as the maximum number of AWTs, to provide the best balance between AWT overhead and contention and CPU and I/O usage. Historically, 80 has worked well as a number that makes available a reasonable number of AWTs for all the different work types, and yet supports up to 40 or 50 new tasks per AMP comfortably. However, managing AWTs is not always a solution to increased demands on the DBS 100. In some cases, an increased demand on system resources may have an underlying cause, such that simply increasing the number of available AWTs may only serve to temporarily mask, or even worsen the demand on resources.

For example, one of the manifestations of resource exhaustion is a lengthening queue for processes waiting for AWTs. Therefore, performance may degrade coincident with a shortage of AWTs. However, this may not be directly attributable to the number of AWTs defined. In this case, adding AWTs will tend to aggravate, not reduce, performance issues.

Using all 80 AWTs in an on-going fashion is a symptom that resource usage is being sustained at a very demanding level. It is one of several signs that the platform may be running out of capacity. Adding AWTs may be treating the effect, but not helping to identify the cause of the performance problem. On the other hand, many Teradata database systems will reach 100% CPU utilization with significantly less than 50 active processes of the new work type. Some sites experience their peak throughput when 40 AWTs are in use servicing new work. By the time many systems are approaching the limit of 80 AWTs, they are already at maximum levels of CPU or I/O usage.

In the case where the number of AWTs is reaching their limit, it is likely that a lack of AWTs is merely a symptom of a deeper underlying problem or bottleneck. Therefore, it is necessary to carry out a more thorough investigation of all events in the DBS 100, in an attempt to find the true source of any slowdowns. For example, the underlying or “real” reason for an increase in CPU usage or an increase in the number of AWTs may be a hardware failure or an arrival rate surge.

Another issue that can impact system-wide performance is a workload event, such as the beginning or conclusion of a load or another maintenance job that can introduce locks or other delays into the DBS 100 or simply trigger the need to change the workload management scheme for the duration of the workload event. The DBS 100 provides a scheduled environment that manages priorities and other workload management controls in operating “windows” that trigger at certain times of the day, week, and/or month, or upon receipt of a workload event.

To manage workloads among these dynamic, system-wide situations, it is important to firstly classify the types of various system events that can occur in a DBS 100, in order to better understand the underlying causes of inadequate performance. As shown in FIG. 8, a plurality of conditions and events are monitored (block 800) and then identified (block 805) so that they can be classified into at least 2 general categories:

-   -   1. System Conditions (block 810), i.e., system availability or         performance conditions; and     -   2. Operating Environment Events (block 815).

System Conditions 810 can include a system availability condition, such as a hardware component failure or recovery, or any other condition monitored by a TASM monitored queue. This may include a wide range of hardware conditions, from the physical degradation of hardware (e.g., the identification of bad sectors on a hard disk) to the inclusion of new hardware (e.g., hot swapping of CPUs, storage media, addition of I/O or network capabilities, etc) It can also include conditions external to the DBS 100 as relayed to the DBS 100 from the enterprise, such as an application server being down, or a dual/redundant system operating in degraded mode.

System Conditions 810 can also include a system performance condition, such as sustained resource usage, resource depletion, resource skew or missed Service Level Goals (SLGs).

An example of a system performance condition is the triggering of an action in response to an ongoing use (or non-use) of a system resource. For example, if there is low sustained CPU and IO for some qualifying time, then a schedule background task may be allowed to run. This can be achieved by lifting throttle limits, raising priority weights and/or other means. Correspondingly, if the system returns to a high sustained use of the CPU and IO, then the background task is reduced (e.g., terminated, priority weights lowered, throttle limits lowered, etc).

Another example of a system performance condition is where a condition is detected due to an increase in the time taken to process a given individual request or workload group. For example, if the average response time is greater than the SLG for a given time interval, then there may be an underlying system performance condition.

Yet another example may be a sudden increase in the number of AWTs invoked (as described earlier).

In other words, system performance conditions can include the following:

-   -   1. Any sustained high or low usage of a resource, such as high         CPU usage, high IO usage, a higher than average arrival rate, or         a high concurrency rate;     -   2. Any unusual resource depletion, such as running out of AWTs,         problems with flow control, and unusually high memory usage;     -   3. Any system skew, such as overuse of a particular CPU in a CPU         cluster, or AWT overuse in a AWT cluster; and     -   4. Missed SLGs.

The second type of detection is an Operating Environment Event 815. Such events can be predetermined or scheduled, in that a user or administrator of the system predefines the event at some point during the operation of the DBS 100. However, in some instances, Operating Environment Events 815 can occur without any appreciable notice being given to the DBS 100 or to users. The event may be time based, business event based or based on any other suitable criteria.

Operating Environment Events 815 can also be defined and associated with the beginning and completion of a particular application job. A user-defined event can be sent by the application and received by the DBS 100. This triggers the regulator of the DBS 100 to operate in the ruleset's working values associated with this event. For example, the working values could direct the DBS 100 to give higher priority to workloads associated with month-end processing, or lower priority associated with workloads doing “regular” work, to enable throttles for non-critical work, and enable filters on workloads that interfere with month-end processing reporting consistency such as might happen when data is being updated while it is being reported on.

In another example, a user may define actions associated with the start of a daily load against a table X. This request triggers a phased set of actions:

-   -   1. Upon the “Begin Acquisition Phase” of MultiLoad to Table X;         -   Promote the priority of all queries that involve table X;         -   At the same time, restrict the ability for new queries             involving table X from starting until after the data load is             completed. Do this through delay, scheduling or disallowing             the query upon request;     -   2. Upon completion of the acquisition phase and the beginning of         the “Apply Phase”, previously promoted queries that are still         running are aborted (“Times Up!”);     -   3. Upon completion of data load, lift restrictions on queries         involving table X, and allow scheduled and delayed queries to         resume.

Another example is to allow the user to define and automate ruleset working value changes based on a user-event (rather than resource or time changes). For example, users may want resource allocation to change based on a business calendar that treats weekends and holidays differently from weekdays, and normal processing differently from quarterly or month-end processing.

As these events are generally driven by business or user considerations, and not necessarily by hardware or software considerations, they are difficult to predict in advance.

Thus, upon detection of any of System Conditions 810 or Operating Environments Events 815, one or more actions can be triggered. In this regard, Block 820 determines whether the detected System Conditions 810 or Operating Environments Events 815 are resolvable.

The action taken in response to the detection of a particular condition or event will vary depending on the type of condition or event detected. The automated action will fall into one of four broad categories (as shown in FIG. 8):

-   -   1. Notify (block 825);     -   2. Change the Workload Management Ruleset's Working Values         (block 830);     -   3. Initiate an automated response (block 835); and     -   4. Log the event or condition, if the condition or event is not         recognized (block 840).

Turning to the first possible automated action, the system may notify either a person or another software application/component including, users, the DBA, or a reporting application. Notification can be through one or more notification approaches:

-   -   Notification through a TASM event queue monitored by some other         application (for example, “tell users to expect slow response         times”);     -   Notification through sending an Alert; and/or     -   Notification (including diagnostic drill-down) through         automation execution of a program or a stored procedure.

Notification may be preferable where the system has no immediate way in which to ameliorate or rectify the condition, or where a user's expectation needs to be managed.

A second automated action type is to change the Workload Management Ruleset's working values.

FIG. 9 illustrates how conditions and events may be comprised of individual conditions or events and condition or event combinations, which in turn cause the resulting actions.

The following is a table that represents kinds of conditions and events that can be detected.

Class Type Description Operating (Time) Period These are the current Periods representing Environment intervals of time during the day, week, or Event month. The system monitors the system time, automatically causing an event when the period starts, and it will last until the period ends. User Defined These are used to report anything that (External)* could conceivably change an operating environment, such as application events. They last until rescinded or optionally time out. System Performance DBS 100 components degrade or fail, or Condition and resources go below some threshold for Availability some period of time. The system will do the monitoring of these events. Once detected, the system will keep the event in effect until the component is back up or the resource goes back above the threshold value for some minimal amount of time. User Defined These are used to report anything that (External)* could conceivably change a system condition, such as dual system failures. They last until rescinded or optionally time out.

Operating Environment Events and System Condition combinations are logical expressions of states. The simplest combinations are comprised of just one state. More complex combinations can be defined that combine multiple states with two or more levels of logical operators, for example, given four individual states, e1 through e4:

Operator Levels Logical Expression 0 e1 1 e1 OR e2 1 e1 AND e2 2 (e1 OR e2) AND (e3 OR e4) 2 (e1 AND e2 AND (e3 OR e4))

Combinations cause one more actions when the logical expressions are evaluated to be “true.” The following table outlines the kinds of actions that are supported.

Type Description Alert Use the alert capability to generate an alert. Program Execute a program to be named. Queue Table Write to a (well known) queue table. SysCon Change the System Condition. OpEnv Change the Operating Environment.

As shown in FIG. 10, the DBS 100 has a number of rules (in aggregation termed a ruleset) which define the way in which the DBS 100 operates. The rules include a name (block 1000), attributes (block 1005), which describes what the rules do (e.g., session limit on user Jane) and working values (WVs) (block 1010), which are flags or values that indicate whether the rule is active or not and the particular setting of the value. A set of all WVs for all the rules contained in a Ruleset is called a “Working Value Set (WVS).”

A number of “states” can be defined, each state being associated with a particular WVS (i.e., a particular instance of a rule set). By swapping states, the working values of the workload management ruleset are changed.

This process is best illustrated by a simple example. At FIG. 10, there is shown a particular WVS which, in the example, is associated with the State “X.” State X, in the example, is a state that is invoked when the database is at almost peak capacity, Peak capacity, in the present example, is determined by detecting one of two events, namely that the arrival rate of jobs is greater than 50 per minute, or alternatively, that there is a sustained CPU usage of over 95% for 600 seconds. State X is designed to prevent resources being channeled to less urgent work. In State X, Filter A (block 1015), which denies access to table “Zoo” (which contains cold data and is therefore not required for urgent work), is enabled. Furthermore, Throttle M (block 1020), which limits the number of sessions to user “Jane” (a user who works in the marketing department, and therefore does not normally have urgent requests), is also enabled. State “X” is therefore skewed towards limiting the interaction that user Jane has with the DBS 100, and is also skewed towards limiting access to table Zoo, so that the DBS 100 can allocate resources to urgent tasks in preference to non-urgent tasks.

A second State “Y” (not shown) may also be created. In State “Y”, the corresponding rule set disables filter “A”, and increases Jane's session limit to 6 concurrent sessions. Therefore, State “Y” may only be invoked when resource usage falls below a predetermined level. Each state is predetermined (i.e., defined) beforehand by a DBA. Therefore, each ruleset, working value set and state requires some input from a user or administrator that has some knowledge of the usage patterns of the DBS 100, knowledge of the data contained in the database, and perhaps even knowledge of the users. Knowledge of workloads, their importance, their characteristic is most likely required more so than the same understanding of individual rules. Of course, as a user defines workloads, most of that has already come to light, i.e., what users and requests are in a workload, how important or critical is the workload, etc. A third action type is to resolve the issue internally. Resolution by the DBS 100 is in some cases a better approach to resolving issues, as it does not require any input from a DBA or a user to define rules-based actions.

Resolution is achieved by implementing a set of internal rules which are activated on the basis of the event detected and the enforcement priority of the request along with other information gathered through the exception monitoring process.

Some examples of automated action which result in the automatic resolution of issues are given below. This list is not exhaustive and is merely illustrative of some types of resolution.

For the purposes of this example, it is assumed that the event that is detected is a longer than average response time (i.e., an exception monitor 615 detects that the response time SLG is continually exceed for a given time and percentage). The first step in launching an automated action is to determine whether an underlying cause can be identified.

For example, is the AWT pool the cause of the longer than average response time? This is determined by seeing how many AWTs are being used. If the number of idle or AWTs is very low, the AWT pool is automatically increased to the maximum allowed (normally 80 in a typical Teradata system).

The SLG is then monitored to determine whether the issue has been ameliorated. When the SLG is satisfactory for a qualifying time, the AWT poolsize is progressively decreased until a suitable workable value is found.

However, the AWT pool may not be the cause of the event. Through the measuring of various system performance indicators, it may be found that the Arrival Rate is the cause of decreased performance. Therefore, rather than limiting on concurrency, the DBS 100 can use this information to take the action of limiting the arrival rate (i.e., throttle back the arrival rate to a defined level, rather than allowing queries to arrive at unlimited rates). This provides an added ability to control the volume of work accepted per WD.

Alternatively, there may be some WDs at same or lower enforcement exceeding their anticipated arrival rates by some qualifying time and amount. This is determined by reviewing the anticipated arrival rate as defined by the SLG.

If there are WDs at the same or lower enforcement exceeding their anticipated arrival rates, the WD's concurrency level is decreased to a minimum lower limit.

The SLG is then monitored, and when the SLG returns to a satisfactory level for a qualifying time, the concurrency level is increased to a defined normal level (or eliminated if no concurrency level was defined originally).

If the event cannot be easily identified or categorized by the DBS 100, then the event is simply logged as a “un-resolvable” problem. This provides information which can be studied at a later date by a user and/or DBA, with a view to identifying new and systemic problems previously unknown.

The embodiment described herein, through a mixture of detection and management techniques, seeks to correctly manage users' expectations and concurrently smooth the peaks and valleys of usage. Simply being aware of the current or projected usage of the DBS 100 may be a viable solution to smoothing peaks and valleys of usage. For example, if a user knows that he needs to run a particular report “sometime today,” he may avoid a high usage (and slow response) time in the morning in favor of a lower usage time in the afternoon. Moreover, if the work cannot be delayed, insight into DBS 100 usage can, at the very least, help set reasonable expectations.

Moreover, the predetermined response to events, through the invocation of different “states” (i.e., changes in the ruleset's working values) can also assist in smoothing peaks and valleys of usage. The embodiment described herein additionally seeks to manage automatically to better meet SLGs, in light of extenuating circumstances such as hardware failures, enterprise issues and business conditions.

However, automated workload management needs to act differently depending on what states are active on the system at any given time. Each unique combination of conditions and events could constitute a unique state with unique automated actions. Given a myriad of possible condition and event types and associated values, a combinatorial explosion of possible states can exist, making rule-based automated workload management a very daunting and error-prone task. For example, given just 15 different condition and event types that get monitored, each with a simple on or off value, there can be as many as 2¹⁵=32,768 possible combinations of states. This number only increases as the number of unique condition and event types or the possible values of each monitored condition or event type increases.

A DBA managing the rules-based management system, after identifying each of these many states must also to designate a unique action for each state. The DBA would further need to associate priority to each state such that if more than one state were active at a given time, the automated workload management scheme would know which action takes precedence if the actions conflict. In general, the DBA would find these task overwhelming or even impossible, as it is extremely difficult to get manage such an environment.

To solve this problem associated with automated workload management, or any rule-driven system in general, the present invention introduces an n-dimensional matrix to tame the combinatorial explosion of states and to provide a simpler perspective to the rules-based environment. Choosing two or more well-known key dimensions provides a perspective that guides the DBA to know whether or not he has identified all the important combinations, and minimizes the number of unique actions required when various combinations occur. Given that n<total possible event types that can be active, each unique event or event combination is collapsed into a finite number of one of the n-dimension elements.

In one embodiment, for example, as shown in FIG. 11, a two-dimensional state matrix 1100 may be used, wherein the first dimension 1105 represents the System Condition (SysCon) and the second dimension 1110 represents the Operating Environment Events (OpEnv). As noted above, System Conditions 1105 represent the “condition” or “health” of the system, e.g., degraded to the “red” system condition because a node is down, while Operating Environment Events 1110 represent the “kind of work” that the system is being expected to perform, e.g., within an Interactive or Batch operational environment, wherein Interactive takes precedence over Batch.

Each element 1115 of the state matrix 1100 is a <SysCon, OpEnv> pair that references a workload management state, which in turn invokes a single WVS instance of the workload management ruleset. Multiple matrix 1100 elements may reference a common state and thus invoke the same WVS instance of the workload management ruleset. However, only one state is in effect at any given time, based on the matrix 1100 element 1115 referenced by the highest SysCon severity and the highest OpEnv precedence in effect. However, a System Condition, Operating Environment Event, or state can change as specified by directives defined by the DBA. One of the main benefits of the states matrix 1100 is that the DBA does not specify a state change directly, but must do so indirectly through directives that change the SysCon or OpEnv.

When a particular condition or event combination is evaluated to be true, it is mapped to one of the elements 1115 of one of the dimensions of the matrix 1100. For example, given the condition “if AMP Worker Tasks available is less than 3 and Workload X's Concurrency is greater than 100” is “true,” it may map to the System Condition of RED. In another example, an event of “Monday through Friday between 7 AM and 6 PM” when “true” would map to the Operating Environment Event of OPERATIONAL_QUERIES.

The combination of <RED, OPERATIONAL_QUERIES>, per the corresponding matrix 1100 element 1115, maps to a specific workload management state, which in turn invokes the WVS instance of the workload management ruleset named WVS#21. Unspecified combinations would map to a default System Condition and a default Operating Environment.

Further, a state identified in one element 1115 of the matrix 1100 can be repeated in another element 1115 of the matrix 1100. For example, in FIG. 11, WVS#33 is the chosen workload management rule when the <SysCon, OpEnv> pair is any of: <RED, QUARTERLY_PROCESSING>, <YELLOW, QUARTERLY_PROCESSING> or <RED, END_OF_WEEK_PROCESSING>.

The effect of all this is that the matrix 1100 manages all possible states. In the example of FIG. 11, 12 event combinations comprise 2¹²=4096 possible states. However, the 2-dimensional matrix 1100 of FIG. 11, with 3 System Conditions and 4 Operating Environment Events, yields at the most 4×3=12 states, although less than 12 states may be used because of the ability to share states among different <SysCon, OpEnv> pairs in the matrix 1100.

In addition to managing the number of states, the matrix 1100 facilitates conflict resolution. Suppose that more than one condition or event combination were true at any given time. Without the state matrix 1100, a list of 4096 possible states would need to be prioritized by the DBA to determine which workload management rules should be implemented, which would be a daunting task. The matrix 1100 greatly diminishes this challenge through the prioritization of each dimension.

For example, the values of the System Condition dimension are Green, Yellow and Red, wherein Yellow is more severe, or has higher precedence over Green, and Red is more severe or has higher precedence over Yellow as well as Green. If two condition and event combinations were to evaluate as “true” at the same time, one thereby mapping to Yellow and the other mapping to Red, the condition and event combination associated with Red would have precedence over the condition and event combination associated with Yellow.

Consider the following examples. In a first example, there may be a conflict resolution in the System Condition dimension between “Red,” which has precedence (e.g., is more “severe”) over “Yellow.” If a node is down/migrated, then a “Red” System Condition exists. If a dual system is down, then a “Yellow” System Condition exists. If a node is down/migrated and a dual system is down, then the “Red” System Condition has precedence.

In a second example, there may be a conflict resolution in the Operating Environment Event dimension between a “Daily Loads” event, which has precedence over “Operational Queries” events. At 8 AM, the Operating Environment Event may trigger the “Operational Queries” event. However, if loads are running, then the Operating Environment Event may also trigger the “Daily Loads” event. If it is 8 AM and the loads are still running, then the “Daily Loads” Operating Environment Event takes precedence.

Once detected, it is the general case that a condition or event status is remembered (persists) until the status is changed or reset. However, conditions or events may have expiration times, such as for user-defined conditions and events, for situations where the status fails to reset once the condition or event changes. Moreover, conditions or events may have qualification times that require the state be sustained for some period of time, to avoid thrashing situations. Finally, conditions or events may have minimum and maximum duration times to avoid frequent or infrequent state changes.

In summary, the state matrix of the present invention has a number of advantages. The state matrix introduces simplicity for the vast majority of user scenarios by preventing an explosion in state handing through a simple, understandable n-dimensional matrix. To maintain this simplicity, best practices will guide the system operator to fewer rather than many SysCon and OpEnv values. It also maintains master control of WVS on the system, but can also support very complex scenarios. In addition, the state matrix can alternatively support an external “enterprise” master through user-defined functions and notifications. Finally, the state matrix is intended to provide extra dimensions of system management using WD-level rules with a dynamic regulator.

A key point of the matrix 1100 is that by limiting actions to only change SysCon or OpEnv (and not states, or individual rules, or rules' WVs), master control is contained in a single place, and avoids having too many entities asserting control. For example, without this, a user might change the individual weight of one workload to give it highest priority, without understanding the impact this has on other workloads. Another user might change the priority of another workload to be even higher, such that they overwrite the intentions of the first user. Then, the DBS 100 internally might have done yet different things. By funneling all actions to be associated with a SysCon or OpEnv instead of directed to individual rules in the ruleset, or directly to a state as a whole, the present invention avoids what could be chaos in the various events. Consequently, in the present invention, the WVS's are changed as a whole (since some settings must really be made in light of all workloads, not a single workload or other rule), and by changing just SysCon or OpEnv, in combination with precedence, conflict resolution is maintained at the matrix 1100.

However, while specific embodiments of a broader invention have been described herein, the present invention may also be carried out in a variety of alternative embodiments and thus is not limited to those described here. For example, while the invention has been described here in terms of a DBS that uses a massively parallel processing (MPP) architecture, other types of database systems, including those that use a symmetric multiprocessing (SMP) architecture, are also useful in carrying out the invention. Many other embodiments are also within the scope of the following claims. 

1. A computer-implemented method for managing workloads in a computer system, comprising: regulating, in the computer system, execution of one or more queries using arrival rate throttles in conjunction with concurrency throttles for a particular workload, wherein the arrival rate throttles manage arrival rates by identifying how many of the queries in the particular workload are executed in a specified amount of time and the concurrency throttles manage concurrency levels by identifying how many of the queries in the particular workload are executed concurrently, wherein the regulated execution of the queries results in the queries being performed by the computer system to access data stored in a database managed by the computer system, and the accessed data being presented for display to a user.
 2. The method of claim 1, wherein the arrival rate throttles provide a capability to throttle back and forward the execution of the queries so as to manage workload concurrency levels.
 3. The method of claim 1, wherein the regulating step queues the queries before execution and releases the queries for execution using the arrival rate throttles.
 4. The method of claim 1, wherein the arrival rate throttles identify how many queries are executed for the particular workload in a specified amount of time.
 5. A computer-implemented apparatus for managing workloads in a computer system, comprising: means for regulating, in the computer system, execution of one or more queries using arrival rate throttles in conjunction with concurrency throttles for a particular workload, wherein the arrival rate throttles manage arrival rates by identifying how many of the queries in the particular workload are executed in a specified amount of time and the concurrency throttles manage concurrency levels by identifying how many of the queries in the particular workload are executed concurrently, wherein the regulated execution of the queries results in the queries being performed by the computer system to access data stored in a database managed by the computer system, and the accessed data being presented for display to a user.
 6. The apparatus of claim 5, wherein the arrival rate throttles provide a capability to throttle back and forward the execution of the queries so as to manage workload concurrency levels.
 7. The apparatus of claim 5, wherein the means for regulating queues the queries before execution and releases the queries for execution using the arrival rate throttles.
 8. The apparatus of claim 5, wherein the arrival rate throttles identify how many queries are executed for the particular workload in a specified amount of time.
 9. An article of manufacture comprising a storage device tangibly embodying instructions that, when executed by a computer system, result in the computer system performing a method for managing workloads in the computer system, the method comprising: regulating, in the computer system, execution of one or more queries using arrival rate throttles in conjunction with concurrency throttles for a particular workload, wherein the arrival rate throttles manage arrival rates by identifying how many of the queries in the particular workload are executed in a specified amount of time and the concurrency throttles manage concurrency levels by identifying how many of the queries in the particular workload are executed concurrently, wherein the regulated execution of the queries results in the queries being performed by the computer system to access data stored in a database managed by the computer system, and the accessed data being presented for display to a user.
 10. The article of claim 9, wherein the arrival rate throttles provide a capability to throttle back and forward the execution of the queries so as to manage workload concurrency levels.
 11. The article of claim 9, wherein the regulating step queues the queries before execution and releases the queries for execution using the arrival rate throttles.
 12. The article of claim 9, wherein the arrival rate throttles identify how many queries are executed for the particular workload in a specified amount of time. 